This invention relates generally to cathode ray tubes (CRTs) and is particularly directed to apparatus and method for controlling image size, geometry and/or position. In an in-line color CRT, the invention may also be employed for correcting horizontal convergence.
As used herein, the term "video" is used in a general sense to apply to any CRT image or picture, as might be developed by a computer monitor, television receiver or special purpose CRT display.
The standard gun/yoke system used in most color computer monitors and television entertainment sets of today includes an in-line electron gun in combination with a self-converging yoke. The in-line electron gun has a built-in static convergence mechanism (offset apertures or angled grid faces, etc.), whose function is to converge the outer beams at the center of the phosphor screen. If a uniform yoke field is applied to deflect the beams to the corners of the screen, the point of convergence falls short, resulting in over-convergence of the outer beams. It is the function of the self-converging yoke to apply a quadrupole-like convergence correction field to keep the beams in convergence as they are deflected.
The quadrupole-like correction field has a well-known adverse effect on the focus characteristics (spot size and shape) of all three beams. A microscopic view shows this field applying a horizontal underconverging force and a vertical overconverging force to each beam bundle causing a horizontal underfocusing action and a vertical overfocusing action. The quadrupole-like correction field causes a deflection defocusing beam condition that increases with deflection. When the beam is in the top left corner, it receives the largest amount of beam distortion; such distortion can only be partially corrected.
A color CRT display such as employed in a television receiver is typically constructed as shown in simplified, schematic form in FIG. 1. The CRT 10 includes an evacuated envelope or bulb, containing red (R), green (G) and blue (B) electron guns at one end directing a plurality of electron beams 13 on a display screen or faceplate 16 at the other end of the CRT. The three electron gun cathodes for R, G and B are horizontally arranged in a tube neck portion 11 and the electron beams 13 emitted therefrom are deflected by a uniform field horizontal deflection coil 14 and produce beam spots on the phosphor-coated inner surface of the display screen 16. A static convergence magnet assembly 12 disposed about the tube neck portion 11 applies magnetic fields to the three electron beams 13 to compensate for electron gun misalignment and to insure convergence of the beams when undeflected. Vertical deflection coils (not shown) are also provided for vertically displacing the electron beams during each horizontal sweep. The three electron beams converge on a "surface of perfect convergence" which is approximately spherical and intersects the display screen 16 at a point where the undeflected center (green) electron beam is incident thereon.
The in-line arrangement of the three electron guns and the non-spherical curvature of the CRT's display screen 16 cause the three electron beams to sometimes be incident upon different locations on the display screen as the beams are swept horizontally across the screen. This electron beam misconvergence is greatest adjacent the lateral edge portions of the display screen 16 as shown in FIG. 1. FIG. 2 shows the red (R) electron beam positioned to the left of the green (G) and blue (B) electron beams on the left-hand and right-hand portions of the display screen 16. Adjacent a vertical center line of the display screen 16, the three electron beams are converged near the center of the display screen, with the two outer beams diverging as the upper and lower edges of the display screen 16 are approached. Horizontal electron beam misconvergence becomes even greater in the case of a color CRT having a perfectly flat glass faceplate.
As is clear from FIG. 2, conventional television receivers, if not compensated, have a severe pincushion geometrical distortion due to the fact that the cathodoluminescent screen does not lie on the surface of perfect convergence. The problem is much more severe in color cathode ray tubes of the type having a perfectly flat screen.
Conventional pincushion compensation circuits do a satisfactory job of rectifying pincushion geometrical distortion, however in severe cases such as are presented by the flat screen tube alluded to, conventional pincushion circuits may nevertheless leave a residuum of errors which cannot be eliminated at reasonable cost. Further, other geometrical distortions may occur in images produced by color cathode ray tubes which cannot be readily compensated by circuitry or other means.
Further, conventional cathode ray tubes having self-converging yokes are not readily suited for high resolution monitor applications wherein the scan frequencies may be in the order of 64 kilohertz or greater.